Calculating numbers from different nodes

Cool module! I'm not php wizard, and Im sure you get heaps of requests. so i'm sorry about this being another one!

i want to be able to add up the number of nodes within a particular group.

I have 20 Banks and 15 Retail stores. Each of these have their own individual node of a type called "Store". 10 of the banks are from the company "ASB Bank" and 10 from "BNZ Bank". The 15 retail stores are all "Progressive". The bank and Progressive are content types called "Company".

What I want to do is in say the Auckland ASB Bank node, i want to be able to put in how many workers there are. Say 10. I also want a computer field below adding that field to the number of worker fields in the other stores. So it would calculate say 10 workers in the Auckland Store out of 100 workers in 10 banks which is also 10 workers out of 200 from all companies. (I would also want to do it based on which type of company, say banks.)

I don't know how to do this but i'm assuming its not too complicated?

Cheers,
Simon